SUMMARY: We aim to learn how Chlamydia trachomatis secreted effectors direct cellular changes both individually and cooperatively within fruit flies (Drosophila melanogaster). Our preliminary data indicate that expression of chlamydial Tarp in flies leads to both macroscopic (curled bristles) and microscopic (irregular microvilli) developmental defects. The fruit fly model organism contains well defined polarized cells and developmental programs which can be quantitatively evaluated. This level of precision in a polarized cell model allows for the molecular dissection of individual C. trachomatis effectors and assessment of effector cooperation which has not yet been investigated. Known targets of the early C. trachomatis effectors such as actin, Crk adapters, Src tyrosine kinases, and PI3K are conserved in the fruit fly model suggesting cellular changes observed in flies are likely to mimic the effects on human tissues. In the short term, we believe that the fruit fly model will provide mechanistic insight into chlamydial effector directed host pathogen interactions and in the long term may serve as a powerful platform to explore bacterial genes of unknown function. Elucidation of the molecular mechanisms employed by C. trachomatis to initiate a successful infection may provide clues that can be applied to novel therapeutic interventions for this prolific pathogen.